hobkirk

I have a 2007 105 triple FD and a 11-27 cassette. The bike has about 5K miles. I recently found I couldn't shift onto the big chain ring so I adjusted the barrel nut out. That made it better, but then I had to do it again after a week. Since that first adjustment I find when I shift from the large ring to the middle ring it will shift all the way to the small ring (this setup has the "trim" position, so I need to shift 2 or 3 clicks whenever I shift the FD, up or down). Sometimes the chain will drop to the small chain ring when I am riding on the middle chain ring and I shift the RD.

PS - The RD cable snapped at the head a month ago. I only replaced the RD cable, figuring the FD cable gets a lot less wear. Right or wrong?

Thanks

reptilezs

reset/check height and rotation. limit screws and adjust the index/cable tension. set shifter to mid front ring and large rear. adjust tension so the inside cage/plate has 1-.5mm gap between chain. fine tune as necessary or to your riding style

Chris_W

Based on the description, my first guess is that the front derailleur has rotated inwards (towards the rear wheel) a little. Make sure the cage is parallel with the chainrings.

fietsbob

Index brifters, some, have 3 big throws, 1 for each chainring,
is it throwing from outer, past middle to the granny, when you tap the release button,
and the return spring does its thing..?

Al1943

I've known this to happen when there was too much tension on the front shift cable. When you shift toward the middle chainring the recoil from the extra tension causes the derailleur to jump too far. But that would not cause the chain to drop from the middle to the smallest ring when shifting the rear. Shifting to a cross-chained combination can contribute to this problem.
I agree with the others to re-adjust the whole setup. I recommend that you use the instructions for derailleur setup and adjustments found at www.parktool.com. It is important to do all of the steps in sequence.
A lot of STI shifting problems can be relieved or cured by flushing out the shifters with WD40 and by replacing the shift cables and housings. It is just as important to replace the housings as it is to replace the cables.